1. Field of the Invention
The present invention relates to a pickup device for recording information to and reproducing the same from a magneto-optical recording medium, and also relates to a photo detecting unit in use with the pickup device.
2. Description of the Prior Art
A conventional magneto-optical recording/reproducing system detects a magneto-optical signal by using a magneto-optical light splitting element by utilizing a birefringence, such as a 3-beam Wollaston prism, in an optical system with collimator, viz., in the parallel light beams.
FIG. 1 is a diagram showing an arrangement of a conventional magneto-optical recording/reproducing pickup device. A light beam emitted by a light source 1, such as a semiconductor laser device, is converted, by a diffraction grating 2, into at least three spot light beams which will be used for generating a tracking control signal. These light beams are collimated by a collimating lens 3, usually consisting of two lenses joined together. The collimated light beams are converged on a-magneto-optical disk 6 by an objective lens 5. To reproduce information, the plane of polarization is turned in accordance with an inverted magnetization pattern corresponding to information written into a vertically magnetized film, in a recording track.
The light beams reflected by the magneto-optical disk 6 are rendered parallel by the objective lens 5 and returned to a beam splitter 4. The light beams are reflected by the beam splitter 4 toward a Wollaston prism 7. The Wollaston prism 7 separates the received light beams into an S polarized light component, a P polarized light component, and a light component as the combination of the S and P polarized light components. These polarized light components are incident on a photo detecting unit 12 through a route of a reflecting mirror 8, a converging lens 9, a concave lens 10, and a cylindrical lens 11 for obtaining a focusing error signal. The direction of the magnetization of a readout signal surface is determined by comparing the intensities of the P and S polarized light components. A focusing error signal is obtained, by the astigmatic method, from the light component as the combination of the S and P polarized light components. A tracking error signal is obtained by comparing the intensities of both sub-beams of the three beams derived from the diffraction grating. Thus, the control signals are generated for controlling the focusing and tracking directions.
Another conventional optical pickup device in use with a magneto-optical recording/reproducing apparatus containing the optical system with collimator is disclosed in Unexamined Japanese Patent Publication (Kokai) Sho-63-127436. In the publication, the parallel beams emanating from the collimating lens are optical-axis transformed (reflected) by a polarizing beam splitter. The reflecting light beams are focused on the magneto-optical disk through an objective lens. The reflecting light beams from the magneto-optical disk are converted into parallel light beams by the objective lens. The light beams, after passing through the polarizing beam splitter, are separated, by an analyzer, into an S polarized light component, a P polarized light component, and a light component as the combination of the S and P polarized light components. Finally, a magneto-optical signal and other control signals for controlling the focusing and tracking directions are formed.
The conventional optical pickup devices including the optical system with collimator is employed in order to suppress a variation of the splitting characteristics of the polarizing beam splitter 4 and the Wollaston prism 7, and to reduce a degree of deterioration of the magneto-optical signals. The objective lenses used in parallel light beams can be more easily designed and manufactured than those used in divergent light beams. For this reason, the collimating lens 3, usually consisting of two spherical glass-joined lenses, for collimating the divergent light beams, and the converging lens 9 for converging the parallel light beams are indispensable for the conventional pickup devices. Use of those lenses brings about complexity of the construction, increase of the number of the indispensable parts, and increase of the size of the optical pickup device.
To solve the problems, there is proposed an optical pickup device in use with the magneto-optical recording/reproducing apparatus, which is designed on the basis of an optical system without collimator, as shown in FIG. 2 ("O plus E", No. 163, 1993, June, pp94 to 95). In the optical pickup device, divergent light beams emitted from a light source 1 pass through a convex lens 13 where a degree of the divergence of the divergent light beams is reduced. The light beams as left divergent are incident, as an S polarized light, on a plate polarizing beam splitter 14. The divergent light beams reflected by the plate polarizing beam splitter 14 are converged on the recording surface of the magneto-optical disk 6, through an objective lens 15. The reflecting light beams are converted, the objective lens 15, into the convergent light beams which in turn enter the plate polarizing beam splitter 14. In the plate polarizing beam splitter 14, the polarizing film allows part of the S polarized light beams and most of the P polarized light beams to pass therethrough. A half wave plate 17, located on the rear side of the plate polarizing beam splitter 14, turns the direction of polarization by 45.degree. of the light beam. Thereafter, a plate analyzer 18 splits the light beam into an S polarized light beam, a P polarized light beam, and a light beam as the combination of the S and P polarized light beams. These light beams are converted into electrical signals by a photo detecting unit 19.
In the pickup device shown in FIG. 2, to obtain exact information, it is necessary to accurately adjust the angles of the plate polarizing beam splitter 14 and the plate analyzer 18. This makes the assembling work difficult. Further, a accurate control of the thickness of the plate analyzer 18 is required. Accordingly, the manufacturing work is difficult. The half wave plate 17 is provided for turning the plane of polarization by 45.degree. and for disposing the plate analyzer 18 on a plane without rotating along the optical axis. This half wave plate 17 is expensive. Provision of the half wave plate 17 runs counter to the cost reduction.
Additional pickup devices based on the optical system without collimator are disclosed in Unexamined Japanese Patent Publication (Kokai) Hei-5-142419, Hei-5-142420, and Hei-5-142421. A Wollaston prism 21 as illustrated in FIGS. 3A and 3B is used. The optical system of the optical pickup device is as shown in FIG. 3C. The Wollaston prism 21 as a multifunctional Wollaston prism includes a polarizing beam splitting film 21c. The polarizing beam splitting film 21c directs an incident light beam 24, which is emitted from a light source 1, toward the objective lens 15, and allows a reflecting light beam 25, which comes in through the objective lens 15, to pass therethrough. (The polarizing beam splitting film 21c is a multilayer film formed by alternately layering a plural number of dielectric thin films of different refractive indices, and is formed on the incident surface of the Wollaston prism 21.) The Wollaston prism 21 consists of a first prism 21a and a second prism 2lb, both being made of crystalline and joined together along their long faces. A plane including the optical axis of the reflecting light beam 25 coming in through the objective lens 15 (the same thing is correspondingly applied to the optical axis of the incident light beam 24 emitted from the light source 1) and the optic axis 21d of the first prism 21a, is at an angle, not a right angle, to a plane including that optical axis and the optic axis 21e of the second prism 2lb. The Wollaston prism 21 thus constructed is disposed slanted with respect to the optical axis in the optical path of the reflecting light beams as non-parallel light beams, whereby an astigmatism is caused.
The multifunctional Wollaston prism 21 splits the reflecting light beam 25 into P polarized light beams b, S polarized light beams c, and the light beams a as the combination of the S and P polarized light beams (FIGS. 3B). These light beams a, b, and c are received by photo detecting elements 16a, 16b, and 16c, respectively (FIG. 3C). A signal processor 16d compares the intensities of the light beams b and c, thereby reading information contained in the reflecting light beams. The photo detecting element 16a containing a 4-division photo diode is capable of producing a focusing error signal as will be described later.
The multifunctional Wollaston prism 21, which is slanted to the optical axis, causes an astigmatism, and hence substitutes for the combination of the polarizing beam splitter and the cylindrical lens. Use of this prism contributes to reduction of the number of the required parts. In the Wollaston prism 21, which is constructed such that the optic axis 21d of the first prism 21a is oriented at a right angle to the optical axis of the light beam passing therethrough, the image by the light beams emitted from the prism is not blurred.
In the construction using the multifunctional Wollaston prism 21 as shown in FIG. 3C, the pickup device produces an insufficient amount of output power to a recording medium. The axially positioning adjustment is essential to the photo detecting elements 16a, 16b, and 16c. This adjustment is laborious and difficult.
In writing information to and reading the same from a recording medium by the optical pickup device as described above, the objective lens must be exactly positioned in both the focusing direction and the tracking direction. In the magneto-optical disk as the recording medium, a Kerr rotation of the plane of polarization is read in the form of a magneto-optical signal. The magneto-optical signal is weaker than a pit signal for the compact disk.
The size reduction of the magneto-optical disk system is a recent trend in this field of the products. The pickup device in use with the magneto-optical disk system is also under a constant pressure of size reduction. In this circumstance, a unique optical pickup device has been proposed (Unexamined Japanese Patent Publication (Kokai) Hei-4-157647). In the pickup device, the combination of a diffraction grating and a 3-beam Wollaston prism, as already described, is used so as to allow a single photo detecting element to receive the reflecting light beams and to pick up thereof information recorded in the magneto-optical disk (in the form of magneto-optical signals), a focusing error signal indicative of a positional deviation (defocusing quantity) in the focus direction, and a tracking error signal indicative of a positional deviation in the track direction.
In this pickup device, three split light beams, the light beam of the 0-th order of diffraction, and the light beams of .+-.1st order of diffraction are incident on the recording layer of the magneto-optical disk. The reflecting light beams R0, R1, and R2 from the recording layer are applied to the 3-beam Wollaston prism 101 as shown in FIG. 4. The 3-beam Wollaston prism 101 further splits each of these reflecting light beams into three reflecting light beams in a direction perpendicular to the separation of the diffraction grating. Totally nine reflecting light beams R0, R1, R2, R10, R11, R12, R20, R21, and R22 are produced. Of those nine reflecting light beams, five light beams R0, R1, R2, R10, and R20 are detected by a single photo detecting unit 102 (FIG. 5). The result of the detecting is used for generating a focusing error signal, a tracking error signal, and a magneto-optical signal.
The conventional photo detecting unit 102 includes detecting elements 103, 106 and 107 for detecting the light beams R0, R1, and R2 corresponding to those of the 0th order and .+-.1st order of diffraction, which are split by the diffraction grating, and detecting faces 104 and 105 for detecting the light beams R10 and R20, which are split by the 3-beam Wollaston prism 101. The photo-detecting elements 103, 106 and 107 for the light beams R0, R1, and R2 are disposed at a right angle to the photo-detecting elements 104 and 105 for the light beams R10 and R20.
As seen, the reflecting light beams R11, R12, R21, and R22, located at four corners, are not used in the conventional pickup device. The ratio of the quantities of the 0th order to .+-.1st order of diffraction, caused by the grating, is set at a relatively small value within a range from 4 to 8, in order to increase the amplitude of the tracking error signal. The 3-beam Wollaston prism of which the basic split-light quantity ratio by the 3-beam Wollaston prism, i.e., ordinary ray:ray as the combination of ordinary ray and extraordinary ray:extraordinary ray, is 25:50:25, is frequently used. The quantity of each of the ordinary and extraordinary rays is the half of that of the combination of the ordinary and extraordinary rays. The magneto-optical signal (ordinary ray intensity--extraordinary ray intensity) is relatively weak.
In the above-mentioned optical pickup device, the reflecting light beams R11, R12, R21, and R22, located at four corners, are not used. Because of this, the tracking error signal is weaker than those by the light beams including those at the four corners. Since the ratio of the quantities of the light of the 0-th order of diffraction to the light of the .+-.1st order of diffraction, caused by the grating, is not large, the resultant focusing error signal and the magneto-optical signal are not large in amplitude. With regard to the basic split-light quantity ratio by the 3-beam Wollaston prism, the beam intensity ratio in the central part is large, while that on both sides is the half of that in the central part. The resultant magneto-optical signal is not large in amplitude.
A cubic beam splitter and a cylindrical lens are usually used in the conventional optical pickup device. The direction of the beam splitting by the grating is at a right angle to that of the beam splitting by the 3-beam Wollaston prism. Accordingly, the photo detecting unit is constructed such that the photo detecting elements for the tracking error signal are arrayed at right angles to the photo detecting elements for the magneto-optical signal.
Use of the cubic beam splitter and a cylindrical lens inevitably increases the number of components and the size of the optical pickup device.
To cope with this, there is proposed an optical pickup device which uses a plate beam splitter with an astigmatism causing function (for generating a focusing error signal) for size reduction purposes ("O plus E", No 163, pp93 and 94). A diffraction grating is not used in this optical pickup system. The conventional photo detecting unit of the type in which the photo detecting elements for the tracking error signal is disposed at a right angle to the photo detecting elements for the magneto-optical signal, is improperly operable when it is applied to the optical pickup system using the diffraction grating and the plate beam splitter. If applied, it fails to produce desired signals.